Electric power steering system having a torque sensor

ABSTRACT

A torque sensor has a torsion bar coaxially in alignment with input and output shafts, a ring shaped magnet fixed to an axial end of the input shaft, a pair of magnetic yokes fixed to an axial end of the output shaft, and a magnetic sensor for detecting magnetic flux density generated between the pair of magnetic yokes. Each of the magnetic yokes is provided with claws, which are circumferentially spaced at constant intervals, and whose number is equal to that of each of N and S poles alternately arranged circumferentially in the magnet. Each center of the claws coincides with a boundary between immediately adjacent N and S poles of the magnet, when the torsion bar is not twisted. The magnetic sensor is inserted into an axial gap between the pair of magnetic yokes without contacting the magnetic yokes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/147,917, filed on May 20, 2002, entitled TORQUE SENSOR AND ELECTRICPOWER STEERING SYSTEM HAVING SAME, which is based upon and claims thebenefit of priority of Japanese Patent Applications No. 2001-148894filed on May 18, 2001, No. 2001-259961 filed on Aug. 29, 2001, No.2001-316435 filed on Oct. 15, 2001 and No. 2001-316788 filed on Oct. 15,2001, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a torque sensor for detecting torqueapplied to a torsion bar to be used in a rotating force transmissionsystem, in particular, in an electric power steering system.

2. Description of Related Art

Conventionally, according to a device disclosed in JP-A-8-159887 fordetecting torsion torque applied to a torsion bar, a magnet and amagnetic sensor are used. The magnet is fixed to an axial end of thetorsion bar and the magnetic sensor is fixed to the other axial end ofthe torsion bar. When the torsion torque is applied to the oppositeaxial ends of the torsion bar, the torsion bar is twisted so that arotation displacement of the magnetic sensor relative to the magnet ischanged. Accordingly, an output responsive to the applied torque isgenerated from the magnetic sensor.

According to the detecting device mentioned above, electric contactssuch as a brush and a slip ring for supplying electric power to andpicking up a signal from the magnetic sensor are necessary, since themagnet and the magnetic sensor are fixed to the opposite axial ends ofthe torsion bar that is rotated. The use of the brush and the slip ringis prone to deteriorate reliability of the detecting device.

Further, according to another detecting device disclosed inJP-A-6-281513, though it is similar to JP-A-8-159887 in view that themagnet and the magnetic sensor are used, helical gears, to which themagnet is fixed, are used for converting the rotation displacement ofthe axial end of the torsion bar relative to the other axial end of thetorsion bar into an axial displacement of the magnet relative to themagnetic sensor that is fixed to a housing. Accordingly, the electriccontacts for supplying electric power to and picking up a signal fromthe magnetic sensor are not necessary.

However, this detecting device uses the gears so that construction ofthe detecting device is complicated. Further, the device has a drawbackin performance since detection errors and response delays seem to beunavoidable due to backrush of the gears and possible wear of the gears.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a torque sensor withoutusing electric contacts, whose construction is more compact and whoseperformance is more accurate.

Another object of the present invention is to provide an electric powersteering system incorporating the torque sensor.

It is an aspect of the present invention to provide a method of easilyassembling a ferromagnetic member to soft magnetic members in the torquesensor.

To achieve any of the above objects, in a torque sensor for detectingtorsion torque to be applied to a first shaft and a second shaft, aresilient member is disposed between and fixed to the first and secondshafts so that the first shaft, the resilient member and the secondshaft are coaxially in alignment with one another. The resilient memberbeing resiliently twisted, when torsion torque is applied to the firstshaft and the second shaft. A ferromagnetic member is connected to oneof a given position of the first shaft and a given position of theresilient member on a side of the first shaft and rotatable togethertherewith. The ferromagnetic member produces a magnetic field. A softmagnetic member is connected to one of a given position of the secondshaft and another given position of the resilient member on a side ofthe second shaft and rotatable together therewith. The soft magneticmember is positioned within the magnetic field and forms a magneticcircuit so that magnetic flux density generated in the magnetic circuitis varied when rotation displacement of the soft magnetic memberrelative to the ferromagnetic member is changed according to the twistof the resilient member. A magnetic sensor is positioned in a vicinityof and without contacting the soft magnetic member for detecting themagnetic flux density generated in the magnetic circuit.

With the torque sensor mentioned above, the magnetic sensor does notdetect directly the magnetic flux generated from the ferromagneticmember. Accordingly, the magnetic sensor can be fixed, for example, to ahousing where the torque sensor is accommodated without the electriccontacts so that reliability of the torque sensor is higher.

It is preferable to have an auxiliary soft magnetic member having amagnetic flux collective portion in the torque sensor. The auxiliarysoft magnetic member is positioned in a vicinity of the soft magneticmember for introducing magnetic flux from the soft magnetic member andconcentrating the same to the magnetic flux collective portion.Accordingly, the magnetic sensor detects the magnetic flux densitygenerated in the magnetic circuit through the magnetic flux collectiveportion. As the magnetic flux generated in the auxiliary soft materialmember is concentrated to the magnetic flux collective portion, themagnetic sensor can detect average of the magnetic flux densitygenerated over an entire circumference of the soft magnetic member.Accordingly, detecting errors are hardly caused by manufacturing errors,assembly inaccuracy of the components constituting the magnetic circuitor misalignment between the first and second shafts.

Preferably, the ferromagnetic member is a ring shaped magnet having Nand S poles alternately arranged circumferentially and the soft magneticmember is a pair of ring shaped magnetic yokes that are positionedaround an outer circumference of the magnet and axially opposed to eachother with an axial gap therebetween. Each of the magnetic yokes hasclaws which are radially spaced at constant intervals and whose numberis equal to that of each of the N and S poles. Further, the claws of oneof the magnetic yokes axially extend toward and are positioned toalternate circumferentially with those of the other of the magneticyoke. The magnetic sensor is positioned in the axial gap between thepair of the magnetic yokes.

With the construction mentioned above, when the angular position of themagnet relative to the magnetic yokes when the resilient member istwisted, the claws of one of the magnetic yokes come closer to the N orS poles and the claws of the other of the magnetic yokes come closer tothe S or N poles. Polarity of magnetic flux flowing in the one of themagnetic yokes is opposite to that in the other of the magnetic yokes.Passive or negative magnetic flux density, which is substantiallyproportional to a twist amount of the resilient member, is generatedbetween both the magnetic yokes.

Further, it is preferable that the auxiliary soft magnetic member is apair of ring shaped auxiliary magnetic yokes each having the magneticflux collective portion. One of the auxiliary magnetic yoke ispositioned around an outer circumference of the one of the magnetic yokeand the other of the auxiliary magnetic yoke is positioned around anouter circumference of the other of the magnetic yoke so that themagnetic flux collective portions of the pair of the auxiliary magneticyokes are axially opposed to each other with an axial gap therebetween.In this case, the magnetic sensor is positioned in the axial gap betweenthe magnetic flux collective portions.

Furthermore, it is preferable that a length of the axial gap betweenboth the magnetic flux collective portions is shorter than that betweenboth portions of the pair of auxiliary magnetic yokes other than themagnetic flux collective portions. This construction serves to improvedetection accuracy of the torque sensor.

As an alternative, the torque sensor may have a first rotationtransmitting member through which the magnet is connected to the one ofthe given position of the first shaft and the given position of theresilient member on a side of the first shaft and a second rotationtransmitting member through which the soft magnetic member is connectedto the other of the given position of the second shaft and the givenposition of the resilient member on a side of the second shaft. In thiscase, the magnet and the pair of magnetic yokes are positioned axiallyin parallel with the resilient member.

Preferably, the first rotation transmitting member is a first gear fixedto the first shaft and a second gear fixed to the magnet, the first andsecond gears being in mesh with each other, and the second rotationtransmitting member is a third gear fixed to the second shaft and afourth gear fixed to the magnetic yokes, the third and fourth gears arebeing in mesh with each other.

With this construction, a sensing portion such as the ferromagneticmember, soft magnetic member and the magnetic sensor can be assembledseparately from the first and second shafts and the resilient member.Accordingly, it is simpler to assemble the sensing portion, for example,to the electric power steering system. Further, the sensing portion canbe replaced as a single body, which facilitates maintenance operation.

Further preferably, each axial center of the claws of both the magneticyokes is positioned to substantially coincide with a boundary betweenimmediately adjacent N and S poles of the magnet, when a twist angle ofthe resilient member shows a reference value. When the resilient memberis not twisted, that is, when the torsion torque is not applied to thefirst and second shafts, if the axial center of the claws is set tosubstantially coincide with a boundary between immediately adjacent Nand S poles of the magnet, the torque sensor is less influenced bymagnetization whose value is lowered due to temperature change.

If two magnet sensors whose magnetism detecting directions are oppositeto each other are used and, preferably are positioned symmetrically withrespect to an axis of the soft magnetic member, difference of theoutputs between the two sensors can be used to cancel temperature driftof the magnet, the magnetic yokes and the magnetic sensor and thesensibility of the torque sensor is doubled.

As an alternative, the magnet sensor may be more than two sensors whichare positioned circumferentially at constant intervals and whosemagnetism detecting directions are same to one another. If the outputsfrom the sensors are processed through adding or average calculation,the detecting accuracy of the torque sensor is remarkably improvedwithout since dimensional fluctuation of magnetic circuit componentssuch as the magnet and the magnetic yokes and position fluctuation ofthe magnetic sensors are less influenced.

It is preferable that a magnetic seal covers at least outercircumference of the magnetic sensor. The magnetic seal serves toeliminate influences of terrestrial magnetism and magnetic fieldsgenerated around the torque sensor so that the erroneous detection isavoided. The magnetic seal may cover only an outer circumference of themagnetic sensor or an entire portion of the magnetic circuit of thetorque sensor.

Preferably, axial length of the magnet is longer than that of themagnetic yoke. Iron filings can be stuck to edges of the magnet withoutentering into a radial gap between the magnet and the magnetic yokes,which does not adversely affect on the magnetic circuit for detectingthe torque so that the erroneous detection may be avoided.

In case that the torque sensor mentioned above is incorporated into anelectric power steering system for steering a vehicle wheel, one of thefirst and second shafts is connected to one end of the steering to whichsteering torque is applied, the other of the first and second shafts isconnected to a steering power transmission mechanism and an electricmotor gives a drive force to the steering power transmission mechanismin response to a control current from a control circuit in response to adetected output of the magnetic sensor for assisting the steering torqueapplied to the steering.

If the magnetic sensor is hole IC, the torque sensor is compact andinexpensive since auxiliary circuits such as a gain adjusting circuit,an offset adjusting circuit and a temperature compensation circuit arenot necessary, so the torque sensor can be composed of less number ofcomponents. Further, since the hole IC does not require an oscillatingcircuit so that noises are hardly radiated, the hole IC does not give anoise problem to surrounding electric devices.

It is preferable that the control circuit has a plate board on which themagnetic sensor is simultaneously mounted. In this case, wire harnessesand connectors for connecting the torque sensor and the control circuitare not necessary, which results in cost saving and better reliabilitybecause of no electric contacts.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will beappreciated, as well as methods of operation and the function of therelated parts, from a study of the following detailed description, theappended claims, and the drawings, all of which form a part of thisapplication. In the drawings:

FIG. 1 is an exploded perspective view of a torque sensor according to afirst embodiment of the present invention;

FIG. 2A is a cross sectional view of the torque sensor of FIG. 1;

FIG. 2B is a plan view of a magnet and magnetic yokes of the torquesensor of FIG. 1;

FIG. 2C is an elevation view of the magnet and the magnetic yokes of thetorque sensor of FIG. 1;

FIG. 3A is a perspective view of a torque sensor according to amodification of the first embodiment;

FIG. 3B is an exploded perspective view of the torque sensor of FIG. 3A;

FIG. 4A is a schematic view of the magnet and the magnetic yokes when atorsion bar is twisted in a direction according to the first embodiment;

FIG. 4B is a schematic view of the magnet and the magnetic yokes whenthe torsion bar is not twisted according to the first embodiment;

FIG. 4C is a schematic view of the magnet and the magnetic yokes whenthe torsion bar is twisted in another direction according to the firstembodiment;

FIG. 4D is a chart showing a relationship between magnetic flux densityand twist angle of the torsion bar according to the first embodiment;

FIG. 5 is an exploded perspective view of a torque sensor according to asecond embodiment of the present invention;

FIG. 6 is a cross sectional view of the torque sensor of FIG. 5;

FIG. 7 is an exploded perspective view of a part of a torque sensoraccording to a third embodiment of the present invention;

FIG. 8 is an exploded perspective view of a part of a torque sensoraccording to a fourth embodiment of the present invention;

FIG. 9 is an exploded perspective view of a part of a torque sensoraccording to a fifth embodiment of the present invention;

FIG. 10 is an exploded perspective view of a part of a torque sensoraccording to a sixth embodiment of the present invention;

FIG. 11 is a chart showing a relationship between magnetic flux densityand magnetic or mechanical angle of the torsion bar according to aseventh embodiment of the present invention;

FIG. 12 is a cross sectional view of a torque sensor according to aneighth embodiment of the present invention;

FIG. 13 is a cross sectional view of a torque sensor for a purpose ofcomparing with the torque sensor according to the eighth embodiment;

FIG. 14 is a plan view of a torque sensor according to a ninthembodiment of the present invention;

FIG. 15 is a cross sectional view of the torque sensor of FIG. 14;

FIG. 16 is a cross sectional view of a torque sensor according to amodification of the ninth embodiment;

FIG. 17 is a schematic view of an entire electric power steering systemaccording to a tenth embodiment of the present invention;

FIG. 18 is a cross sectional view of a torque sensor according to aneleventh embodiment of the present invention;

FIG. 19 is a cross sectional view of a torque sensor according to atwelfth embodiment of the present invention;

FIG. 20A is a perspective view of a torque sensor according to athirteenth embodiment of the present invention;

FIG. 20B is a plan view of a rotation transmission member of the torquesensor of FIG. 20A;

FIG. 21A is a cross sectional view of a torque sensor mounted on acolumn housing according to a fourteenth embodiment of the presentinvention;

FIG. 21B is a schematic view of the torque sensor of 21A as viewedaxially;

FIG. 22A is a cross sectional view of a torque sensor mounted on acolumn housing according to a modification of the fourteenth embodiment;and

FIG. 22B is a schematic view of the torque sensor of FIG. 22A as viewedaxially.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

A torque sensor 1 according to a first embodiment is described withreference to FIGS. 1 to 4D.

FIG. 1 shows an exploded perspective view of the torque sensor 1. FIG.2A is a cross sectional view of the torque sensor 1. FIG. 2B and 2C showplan and elevation views of a magnet and magnetic yokes, respectively.

The torque sensor 1 is applicable, for example, to an electric powersteering system for a vehicle (refer to FIG. 17) and disposed between aninput shaft 2 and an output shaft 3 that constitute a steering shaft.The torque sensor 1 is for detecting steering torque applied to thesteering shaft.

The torque sensor 1 is composed of a torsion bar 4 (resilient member)connecting coaxially the input shaft 2 and the output shaft 3, a magnet5 (ferromagnetic member) fixed to an axial end of the input shaft 2 on aside of the output shaft 3, a pair of magnetic yokes 6 (soft magneticmember) fixed to an axial end of the output shaft 3 and a magneticsensor 7 for detecting magnetic flux density generated between the pairof the magnetic yokes 6.

Opposite axial ends of the torsion bar 4 are inserted into holes of theinput and output shafts 2 and 3, respectively, and fixed via pins 8 tothe other axial end of the input shaft 2 and the other axial end of theoutput shaft 3, respectively. The torsion bar 4 has given torsion/torquecharacteristics necessary for bringing an adequate rotating displacementof the axial end thereof relative to the other axial end thereof.Accordingly, when the torsion bar 4 is twisted, the axial end of theinput shaft 2 can be rotated, or circumferentially shifted, relative tothe axial end of the output shaft 3.

The magnet 5, which is formed in a ring shape and composed of N and Spoles that are alternately arranged in a circumferential directionthereof, is positioned outside an outer circumference of the torsion bar4. The magnet 5 has, for example, 24 poles.

As shown in FIG. 1, each of the pair of magnetic yokes 6 (6A, 6B) isformed in a ring shape and arranged around and in a vicinity of an outercircumference of the magnet 5. Each of the magnetic yokes 6A or 6B isprovided with claws 6 a which are spaced circumferentially at constantintervals and whose number is equal to that of N or S poles of themagnet 5 (12 pieces). The pair of the magnetic yokes 6 are fixed to andsupported by a fixing base or holder 9 (refer to FIG. 2) in such amanner that the claws 6 a of the magnetic yoke 6A and the claws 6 a ofthe magnetic yoke 6B extend axially in a direction of coming close toeach other and are positioned to circumferentially alternate with oneanother.

In a state that the torsion bar 4 is not twisted (when torsion torque isnot applied to the torsion bar 4 to cause the input shaft 2 to rotaterelative to the output shaft 3), each axial center of the claws 6 a ofthe magnetic yokes 6 (6A and 6B) is positioned to coincide with aboundary between immediately adjacent N and S poles of the magnet 5.

As more clearly shown in FIG. 2C, the magnetic sensor 3 is positioned inan axial gap G existing between the magnetic yoke 6A and the magneticyoke 6B and detects the magnetic flux density generated between themagnetic yokes 6A and 6B. The magnetic sensor 3 is fixed to a givenposition of a housing (not shown) without coming in contact with themagnetic yokes 6.

The magnetic sensor 7 has a hole element, a hole IC or a magneticresistance element which outputs an electric signal (for example,voltage signal) whose value is converted from a value of the detectedmagnetic flux density.

Though the magnet 5 is fixed to the axial end of the input shaft 2 andthe magnetic yokes 6 are fixed to an axial end of the output shaft 3 inthe above embodiment, as shown in FIGS. 1 to 2C, the magnet 5 may befixed to the axial end of the torsion bar 4 on a side of the input shaft2 and the magnetic yokes 6 may be fixed to the other axial end of thetorsion bar 4 on a side of the output shaft 3, as shown in FIGS. 3A and3B. In this case, an inner surface of the ring shaped magnet 5 is pressfitted to an outer surface of the torsion bar 4 and an inner hole of aholder 9A for supporting the ring shaped magnetic yokes 6A and 6B ispress fitted to the outer surface of the torsion bar 4.

The magnet 5 and the magnetic yokes 6 are assembled in the torque sensor1 and the magnet 5 is positioned relative to the magnetic yokes 6through the following steps;

(1) fixing the ring shaped magnetic yokes 6 to the torsion bar 4 on aside of the output shaft 3 or the axial end of the output shaft 3, forexample, by press fitting or gluing,

(2) inserting the magnet 5 into the ring shaped magnetic yokes 6 andholding the magnet 5 therein so as to allow a free rotation relative tothe torsion bar 4 or the input shaft 2 for tentative assembly,

(3) defining a magnet rest position where the magnet 5 rests within thering shaped magnet yokes 6 due to magnetic attracting force generatedbetween the magnet 5 and the ring shaped magnet yokes 6, and

(4) fixing the magnet 5 to the torsion bar 4 on a side of the inputshaft 2 or the axial end of the input shaft 2 in such a manner that themagnet 5 maintains the magnet rest position.

An operation of the torque sensor 1 is described hereinafter.

As shown in FIG. 4B, in the state that torque is not applied to thetorsion bar 4 so that the input shaft 2 is not rotated relative to theoutput shaft 3, that is, at a neutral position that the torsion bar 4 isnot twisted, each axial center of the claws 6 a of the magnetic yokes 6coincides with a boundary between the immediately adjacent N and S polesof the magnet 5. In this case, since the number of lines of magneticforce passing between each of N poles and each of the claws 6 a is equalto that passing between the each of the claws 6 a and each of S poles,the lines of magnetic force are closed within the respective magneticyokes 6A and 6B and do not leak into the axial gap G between themagnetic yoke 6A and the magnetic yoke 6B. Accordingly, the value of themagnetic flux density to be detected by the magnetic sensor 7 is zero,as shown in FIG. 4D.

As shown in FIG. 4A or 4C, in a state that torque is applied to thetorsion bar 4 so that the input shaft 2 is rotated relative to theoutput shaft 3, that is, when the torsion bar 4 is twisted, an angularposition of the magnet 5, which is fixed to the input shaft 2, relativeto the pair of magnetic yokes 6, which are fixed to the output shaft 3,is circumferentially changed. As the each axial center of the claws 6 aof the magnetic yokes 6 is circumferentially shifted from a boundarybetween the respective N and S poles of the magnet 5, the number oflines of magnetic force having N or S pole increases in each of themagnetic yokes 6A and 6B. Since polarity of the lines of magnetic forcewhose number increases in one of the magnetic yokes 6A is opposite tothat in the other of the magnetic yokes 6B, the magnetic flux density isgenerated between the magnetic yokes 6A and 6B, that is, in the axialgap G. The value of the magnetic flux density is about proportional to atwisted amount of the torsion bar 4 and its polarity can be invertedaccording to the direction in which the torsion bar 4 is twisted.

Advantages of the torque sensor 1 according to the first embodiment aredescribed hereinafter.

When the torsion bar 4 is twisted and the relative position of themagnet 5 to the pair of magnetic yokes 6 is circumferentially changed,the magnetic flux density between the pair of magnetic yokes 6 ischanged at an entire circumference thereof and the value of the magneticflux density is identical at any circumferential position. Accordingly,if the magnetic sensor 7 is position at a given position in the axialgap G with which the magnetic yokes 6A and 6B are opposed to each other,the magnetic sensor 7 can detect the magnetic flux density between thepair of magnetic yokes 6 without contacting the magnetic yoke 6.Therefore, detecting reliability of the torque sensor 1 is higher sincethe electric contacts (for example, the brush and the slip ring) for themagnetic sensor 7 are not necessary.

Further, since the each axial center of the claws 6 a of the magneticyokes 6 coincides with a boundary between the immediately adjacent N andS poles of the magnet 5, when the torsion bar 4 is not twisted, aneutral point of the magnetic sensor 7 can be never shifted, even ifmagnetic force of the magnet 5 is changed due to temperature change, asshown in FIG. 4D. Accordingly, the torque sensor 1 is unlikely to beaffected by offset drift and accuracy thereof in a vicinity of theneutral point is more stable.

Furthermore, since, after the magnet rest position is defined, themagnet 5 is fixed to the torsion bar 4 or the input shaft 2, so as tomaintain the magnet rest position, the position of the magnet 5 relativeto the magnetic yokes 6 can be accurately defined so that the output ofthe magnetic sensor is substantially zero, when the torsion bar 4 is nottwisted.

(Second Embodiment)

A torque sensor 1 according to a second embodiment is described withreference to FIGS. 5 and 6.

FIG. 5 shows an exploded perspective view of a torque sensor 1. FIG. 6is a cross sectional view of the torque sensor 1.

The torque sensor 1 according to the second embodiment has a pair ofmagnetic flux collective rings 10 (auxiliary soft magnetic member) inaddition to components of the first embodiment.

Each of the magnetic flux collective rings 10 (10A, 10B) is made of samesoft magnetic material as the magnetic yokes 6 and formed in a ringshape. The magnetic flux collective rings 10A and 10B are positionedaround and in a vicinity of outer circumferences of the magnetic yokes6A and 6B, respectively.

Each of the magnetic flux collective rings 10 is provided at acircumferential position thereof with a flat collective plate 10 a. Thecollective plates 10 a of the magnetic flux collective rings 10A and 10Bare axially opposed to each other. Axial distance between the collectiveplates 10 a is shorter than that between the other parts of the magneticflux collective rings 10A and 10B. The magnetic sensor 7 is positionedbetween the collective plates 10 a axially opposed to each other anddetects magnetic flux density generated between the collective plates 10a.

With the construction mentioned above, magnetic flux generated from themagnet 5 is collected with priority on the collective plates 10 a viathe magnetic yokes 10, since the magnetic flux collective rings 10constitute a part of magnetic circuit. The magnetic sensor 7 detectsmagnetic flux density between the collective plates 10 a, whose value isan average value of the magnetic flux density between the entirecircumferences of the magnetic yokes 6. Accordingly, in the torquesensor 1 according to the second embodiment, detecting errors are hardlycaused by manufacturing or errors, assembly inaccuracy of the componentsconstituting the magnetic circuit or misalignment between the input andoutput shafts 2 and 3.

(Third Embodiment)

A torque sensor 1 according to a third embodiment is described withreference to FIG. 7. FIG. 7 shows an exploded perspective view of a partof the torque sensor 1.

The torque sensor 1 according to the third embodiment has two magneticsensors 2 which are positioned in the axial gap between the magneticyokes 6A and 6B. Magnetism detecting directions of the respectivemagnetic sensors 7 are opposite to each other, as shown in arrows marksin FIG. 7. Each of the magnetic sensors 7 is connected to a differentialcircuit 11. The differential circuit 11 outputs a torque signal afteroutput signals from the magnetic sensors 7, which are input to thedifferential circuit 11, are processed differentially therein.

In case of a single magnetic sensor 7, the detection fluctuation due toa position where the magnetic sensor is located is relatively large.However, as the torque sensor 1 according to the third embodiment hastwo magnetic sensors 7, the detecting fluctuation due to positions wherethe magnetic sensors are located is smaller.

Further, output difference between the magnetic sensors 7 can beeffectively used for canceling temperature drift and increasingdetection sensitivity.

The differential circuit 11 may be or not be a component of the torquesensor 1. Unless the differential circuit is the component of the torquesensor 1, ECU (not shown) plays a roll of the differential circuit 11and may execute differential processes based the output of the magneticsensors 7 for calculating the torque.

The two magnetic sensors according to third embodiment may be applied tothe second embodiment, too.

(Fourth Embodiment)

A torque sensor 1 according to a fourth embodiment is described withreference to FIG. 8. FIG. 8 shows an exploded perspective view of a partof a torque sensor 1.

The torque sensor 1 according to the fourth embodiment has two magneticsensors 7, which is similar to the third embodiment. The two magneticsensors 7 are arranged symmetrically with respect to the torsion bar 4(on radially opposite sides of the torsion bar 4) in the axial gapbetween the magnetic yokes 6A and 6B. Magnetism detecting directions ofthe respective magnetic sensors 7 are opposite to each other, as shownin arrows marks in FIG. 8. Each of the magnetic sensors 7 is connectedto a differential circuit 11. The differential circuit 11 outputs atorque signal after output signals from the magnetic sensors 7, whichare input to the differential circuit 11, are processed differentiallytherein.

As the torque sensor 1 according to the fourth embodiment has twomagnetic sensors 7, which is similar to the third embodiment, thedetection is less affected by positions where the magnetic sensors arelocated and, therefore, the detection accuracy is higher, compared withthat of the single magnetic sensor.

Further, output difference between the magnetic sensors 7 can beeffectively used for canceling temperature drift and increasingdetection sensitivity twice because detection physical quantity isdoubled. Moreover, the misalignment between the input and output shaft 2and 3 is less affected on detecting accuracy.

The differential circuit 11 may be or not be a component of the torquesensor 1. Unless the differential circuit 11 is the component of thetorque sensor 1, ECU (not shown) plays a roll of the differentialcircuit 11 and may execute differential processes based the outputs ofthe magnetic sensors 7 for calculating the torque.

(Fifth Embodiment)

A torque sensor 1 according to a fifth embodiment is described withreference to FIG. 9. FIG. 9 shows an exploded perspective view of a partof a torque sensor 1.

The torque sensor 1 according to the fifth embodiment has two magneticsensors 7 arranged symmetrically with respect to the torsion bar 4 inthe collective plates 10 a of the magnetic flux collective rings 10(10A, 10B), which is similar to the second embodiment. The collectiveplates 10 a according to the fifth embodiment are two pairs ofcollective plates 10 a which are circumferentially spaced at 180°intervals, as shown in FIG. 9.

Each of the two magnetic sensors 7 is positioned between one of thepairs of the collective plates 10 a axially opposed to each other.Magnetism detecting directions of the respective magnetic sensors 7 areopposite to each other, as shown in arrows marks in FIG. 9. Each of themagnetic sensors 7 is connected to a differential circuit 11. Thedifferential circuit 11 outputs a torque signal after output signalsfrom the magnetic sensors 7 are processed differentially therein.

The fifth embodiment has not only an advantage that each of the magneticsensors 7 detects an average value of the magnetic flux density betweenthe entire circumferences of the magnetic yokes 6 because of using themagnetic flux collective rings 10 but also another advantage that thedetection sensitivity is twice and the misalignment between the inputand output shaft 2 and 3 is less affected on detecting accuracy.

(Sixth Embodiment)

A torque sensor 1 according to a sixth embodiment is described withreference to FIG. 10. FIG. 10 shows an exploded perspective view of apart of a torque sensor 1.

The torque sensor 1 according to the sixth embodiment has more than twopieces of magnetic sensors 7 (three pieces of magnetic sensors 7 in thisembodiment).

The three magnetic sensors 7, which are circumferentially spaced atconstant intervals, are arranged in the axial space between the magneticyokes 6A and 6B and connected to a calculation circuit 12. Magnetismdetecting directions of the respective magnetic sensors 7 are same toone another. The calculation circuit 12 outputs a torque signal afterprocessing to add or average outputs of the three magnetic sensors 7.

Since the torque sensor 1 according to the sixth embodiment has threemagnetic sensors 7 and the outputs thereof are processed through addingor average calculation, the detecting accuracy is remarkably improved,compared with that of the single magnetic sensor 7, whose detection ofthe magnetic flux density is largely affected by the position where themagnetic sensor 7 is located.

The calculation circuit 12 may be or not be a component of the torquesensor 1. Unless the calculation circuit 12 is the component of thetorque sensor 1, ECU (not shown) plays a roll of the calculation circuit12 and may execute adding or average processes based the outputs of themagnetic sensors 7 for calculating the torque.

(Seventh Embodiment)

FIG. 11 shows a graph illustrating a relationship between a twist angleof the torsion bar 4 (a displacements angle of the magnet 5 to themagnetic yokes 6) and magnetic flux density generated between themagnetic yokes 6. The twist angle of the torsion bar 4 is shown as amaximum twist angle of the torsion bar 4 in relation with a polar numberof the magnet 5 or the magnetic yokes 6.

As shown in FIG. 11, if the following formula (1) is satisfied, torquecan be accurately detected (sensible zone).θ_(max) ×n≦120[deg].  (1)

where θ_(max) is a maximum twist angle of the torsion bar 4 and n is apolar number of the magnet 5 or the magnetic yokes 6.

Preferably, if the following formula (2) is satisfied, torque can bemore accurately detected since the value of the magnetic flux density ismore linearly changed with respect to the maximum twist angle of thetorsion bar 4 (linear zone).θ_(max) ×n≦60[deg].  (2)(Eighth Embodiment)

A torque sensor 1 according to an eighth embodiment is described withreference to FIG. 12. FIG. 12 shows an exploded perspective view of atorque sensor 1.

The torque sensor 1 according to the eighth embodiment has a magnet 5whose axial length is longer than that of the magnetic yokes 6, as shownin FIG. 12.

For example, in the torque sensor 1 in which an axial length of themagnet 5 is substantially equal to or shorter than that of the magneticyokes 7, a radial gap between the magnet 5 and the magnetic yokes 7 isprone to be filled with iron filings Q, if invaded into the torquesensor 1 from outside, which cause short circuit of the magnetic circuitand, thus, erroneous detection.

However, in a case that opposite axial ends of the magnet 5 axiallyprotrude out of the opposite axial ends of the magnetic yokes 7, asshown in the eighth embodiment, the iron filings Q are stuck to edges ofthe magnet 5 (since the magnet 5 has characteristics that magnetic fluxis concentrated to the edges thereof), which does not adversely affecton the magnetic circuit for detecting the torque so that the erroneousdetection may be avoided.

(Ninth Embodiment)

FIG. 14 shows a plan view of a torque sensor 1 according to a ninthembodiment. FIG. 15 shows a cross sectional view of the torque sensor 1according to the ninth embodiment.

The torque sensor 1 according to the ninth embodiment has a magneticseal 13 (magnetic material) covering a substantially entire portion ofthe magnetic circuit thereof.

The magnetic seal 13 is formed in cylindrical shape, as shown in FIGS.14 and 15. The magnetic seal 13 serves to shut out influences ofterrestrial magnetism and magnetic fields generated around the torquesensor 1 so that the erroneous detection is avoided.

Further, as shown in FIG. 16, the magnetic seal 13 may cover only themagnetic sensor 7 without covering the entire portion of the magneticcircuit of the torque sensor 1.

(Tenth Embodiment)

An electric power steering system incorporating the torque sensor of thepresent invention according to a tenth embodiment is described with FIG.17.

The electric power steering system according to the tenth embodiment iscomposed of an electric motor 15 for giving additional force to asteering power transmission mechanism 14A, which connect the outputshaft 3 and wheels 14B, for assisting the steering torque applied to asteering mechanism such as steering wheel 14 by a driver, the torquesensor 1 for detecting the steering torque applied to the steering wheel14 and a control circuit for controlling current to be supplied to theelectric motor 15 in response to the value of the torque detected by thetorque sensor 1. The construction of the torque sensor 1 is, forexample, same as that of the first embodiment.

As the electric power steering system mentioned above does not have acoil for detecting change of magnetic fields and a coil for compensatingtemperature change, which are provided in a conventional electric powersteering system, a large housing for accommodating these coils is notnecessary.

Further, torque sensor 1 does not emit electric noises with less powerconsumption, since alternating current is not applied to the coil as inthe conventional torque sensor.

The magnetic sensor 7 uses the hole IC which causes the torque sensor 1compact and inexpensive since auxiliary circuits such as a gainadjusting circuit, an offset adjusting circuit and a temperaturecompensation circuit are not necessary, so the torque sensor 1 can becomposed of less number of components.

Further, since the hole IC does not require an oscillating circuit sothat noises are hardly radiated, the hole IC does not give a noiseproblem to surrounding electric devices.

Furthermore, since electric components other than the hole IC are notnecessary, the magnet sensor 7 can be operative with less powerconsumption and at relatively high temperature, which the hole IC canendure for its use.

Moreover, since the gain adjustment, the offset adjustment and thetemperature compensation, which have been executed by ECU in theconventional torque sensor, can be performed within the hole IC, qualityassurance of the torque sensor 1 is available as a single body and, ifthe torque sensor 1 fails, only the failed torque sensor 1 can bereplaced without consulting with the other components such as ECU.Further, it is not necessary to initialize the torque sensor 1, when thetorque sensor 1 is assembled to a torque sensor system, for example, tothe electric power steering system, resulting higher productivity andlower cost.

(Eleventh Embodiment)

As shown in FIG. 18, an electric power steering system according to aneleventh embodiment has a circuit board 17 in which the control circuit16 (refer to FIG. 17) and the magnetic sensor 7 for the torque sensor 1are simultaneously installed. The circuit board 17 is fixed, forexample, with screws to a housing 18 in which the torque sensor 1 isaccommodated.

In this case, wire harnesses and connectors for connecting the torquesensor 1 and the control circuit 16 are not necessary, which results incost saving and better reliability because of no electric contacts.

(Twelfth Embodiment)

As shown in FIG. 19, in an electric power steering system according to atwelfth embodiment, the magnetic sensor 7 is mounted on a connector orplug 21 of a wire harnesses 20 for connecting the torque sensor 1 andthe control circuit 16.

In this case, if the connector 21 with the magnet sensor 7 is simplyinserted into a housing 18 of the torque sensor 1, the assembly of themagnet sensor 7 is simpler.

(Thirteenth Embodiment)

In an electric power steering system according to a thirteenthembodiment, a sensing portion S of the torque sensor 1 can be assembledat a later time. The sensing portion S is composed of a ring shapedmagnet 5, a pair of ring shaped magnetic yokes 6(6A, 6B) and a magneticsensor 7.

As shown in FIG. 20, an input shaft 3, a torsion bar 4 and an outputshaft 3 are axially in alignment with one another. The sensing portion Sis positioned axially in parallel with the torsion bar 4. The inputshaft 2 is connected to the magnet 5 via a first torque transmissionmember that is composed of a gear 22 attached coaxially to the inputshaft 2 and a gear 23 attached coaxially to the magnet 5 a. The gears 22and 23 are in mesh so that rotation of the input shaft 2 is transmittedto the magnet 5. The output shaft 3 is connected to the magnetic yokes 6via a second torque transmission member that is composed of a gear 24attached coaxially to the output shaft 3 and a gear 25 attachedcoaxially to the magnetic yokes 6. The gears 24 and 25 are in mesh sothat rotation of the output shaft 3 is transmitted to the magnetic yokes6.

With the construction mentioned above, the sensing portion S can beassembled separately after the input shaft 2 with the gear 22, thetorsion bar 4, the output shaft 3 with the gear 25, the electric motor15 (refer to FIG. 17) and the steering power transmission mechanism 14A(refer to FIG. 17) are assembled in advance. Accordingly, it is simplerto assemble the sensing portion S to the electric power steering system.Further, the sensing portion S can be replaced as a single body, whichfacilitates maintenance operation.

(Fourteenth Embodiment)

As shown in FIG. 21, an electric power steering system according to afourteenth embodiment has a magnetic seal 26 surrounding the torquesensor 1. The magnetic seal 26 covers entire outer circumference of acolumn housing 27 (for example, made of aluminum) in which the torquesensor 1 is housed.

The torque sensor 1 to be used in the electric power steering system isprone to erroneously detect the torque, if influenced by outsidemagnetic fields generated by, for example, onboard speakers(incorporating magnet members). Accordingly, magnetic sealing around theouter circumference of the torque sensor 1 prevents the torque sensor 1from erroneously detecting due to the outside magnetic fields.

As shown in FIG. 22, in place of magnetic sealing the entire outercircumference of the column housing 27, only a portion of the columnhousing 27 where the magnetic sensor 7 is positioned may be magneticsealed.

In the embodiments mentioned above, instead that the magnet 5 isconnected to the first shaft 2 or the torsion bar 4 on a side of thefirst shaft 2 and the magnetic yoke 6 is connected to the second shaft 3or the torsion bar 4 on a side of the second shaft 3, the magnet 5 maybe connected to the second shaft 3 or the torsion bar 4 on a side of thesecond shaft 3 and the magnetic yokes 6 may be connected to the firstshaft 2 or the torsion bar 4 on a side of the first shaft 2.

Further, to assemble the magnet 5 and the magnetic yokes 6 in the torquesensor 1, after the magnet 5 is fixed at first to the torsion bar 4 orone of the input and output shafts 2 and 3, the magnetic yokes 6 maycover the magnet 5 to define a magnetic yoke rest position and, then,the magnetic yokes 6 may be fixed to the torsion bar 4 or the other ofthe input and output shafts 2 and 3 to maintain the magnetic yokeposition.

1. An electric power steering system for steering a vehicle wheel,comprising: a torque sensor for detecting torsion torque to be suppliedto a first shaft and a second shaft, the torque sensor including aresilient member disposed between and fixed to the first and secondshafts so that the first shaft, the resilient member and the secondshaft are coaxially in alignment with one another, a ferromagneticmember for producing a magnetic field and connected to one of theresilient member and the first shaft, a soft magnetic member connectedto one of the second shaft and the resilient member, the soft magneticmember positioned within the magnetic field and forming a magneticcircuit, and a magnetic sensor for detecting the magnetic flux densitygenerated in the magnetic sensor; a steering mechanism to which steeringtorque is applied, one end of the steering mechanism is connected to oneof the first and second shafts; steering power transmission mechanismwhose one end is connected to the wheel and whose other end is connectedto the other of the first and second shafts; an electric motor connectedto the steering power transmission mechanism and in circuit with themagnetic sensor of the torque sensor; and a control circuit forgenerating control current to be supplied to the electric motor inresponse to a detected output of the magnetic sensor, wherein theelectric motor gives a drive force to the steering power transmissionmechanism in response to the control current for assisting the steeringtorque applied to the steering.
 2. An electric power steering systemaccording to claim 1, wherein the magnetic sensor is hole IC.
 3. Anelectric power steering system according to claim 1, wherein the controlcircuit has a plate board on which the magnetic sensor is simultaneouslymounted.
 4. An electric power steering system according to claim 1,further comprising: a connector of a wire harness connecting the controlcircuit and the torque sensor, wherein the magnetic sensor is mounted onthe connector.
 5. An electric power steering system according to claim1, further comprising: a column housing in which the torque sensor ishoused; and a magnetic seal covering at least an outer circumference ofthe column housing where the magnetic sensor is positioned, the magneticseal preventing an influence from an external magnetic field generatedoutside the torque sensor.
 6. An electric power steering systemaccording to claim 5, wherein the magnetic seal covers only a part ofthe outer circumference in a vicinity of the magnetic sensor.
 7. Anelectric power steering system according to claim 1, wherein: theresilient member is resiliently twisted when torsion torque is appliedto the first shaft and the second shaft; the ferromagnetic member isconnected to one of a given position of the first shaft and a givenposition of the resilient member on a side of the first shaft and isrotatable together therewith; the soft magnetic member is connected toone of a given position of the second shaft and another given positionof the resilient member on a side of the second shaft and in rotatabletogether therewith; the soft magnetic member is positioned within themagnetic field and forming the magnetic circuit so that magnetic fluxdensity generated in the magnetic circuit is varied when rotationdisplacement of the soft magnetic member relative to the ferromagneticmember is changed according to the twist of the resilient member; andthe magnetic sensor is positioned in a vicinity of and withoutcontacting the soft magnetic member.